There has been proposed, in French Patent Specification No. 2,279,992, a seal for a butterfly valve which comprises two sealing elements of a deformable material for sealing against the periphery of the valve disk. A rigid former is mounted between the two sealing elements, to support the elements and control their engagement with the disk, the former being clamped together with the sealing elements between a portion of the valve housing and a clamping ring. The two sealing elements are arranged so that the seal is effective against flow in either direction through the valve.
For some applications, for example for uses in the petrochemical industry, there is a requirement that a rotary valve can in the event of a fire still be closed to prevent flow along a pipeline. In rotary valves it is known to use seals of materials, such as materials based upon polytetrafluoroethylene (P.T.F.E.), which may be destroyed in the event of a serious fire, and provision must accordingly be made for a fireproof back-up seal which would survive a fire of the expected intensity.
Such valves are known as `fire-safe valves` and meet well known standards such as British Standard No. 5146.
With a seal as shown in Specification No. 2,279,992, upon destruction of the sealing elements in a fire, the former, since it is clamped together with the sealing elements, would be left unclamped, and accordingly could not be employed to provide a reliable back-up seal.